Recently, among machine tools such as machining centers etc, fully automatic types allowing unmanned processing are becoming the mainstream and thus many kinds of drills corresponding to the fully automatic types have been developed and sold. However, drills used for devices requiring operator's power to drill holes, such as a hand drill or a drill press, have not been positively researched or developed. As a result, drills in almost same configurations have been used over several decades.
Since the drills used for the hand drill and drill press etc drill holes with the use of operator's arm strength, they have problems in drilling holes if drilling resistance is high. However, manufacturers of drills have spent little time on research for decreasing the high drilling resistance. It is partly because they thought these conventional drills should be firstly assured for their strength and stiffness, and because operators who bought drills actually used to grind by themselves and utilize the drills as they wanted.
Generally, a method for narrowing a chisel width by performing thinning on a cutting blade is known as a method for decreasing drilling resistance of a drill. (For example, see the following Patent Document 1). Usually, conventional thinning was performed from a central part to a heel part of the drill (see FIG. 16(a)), or only narrow area of the drill's central part (see FIG. 16(b)). In the FIGS. 16(a) and (b), a part on which thinning was performed is hatched and a cutting blade shaped by thinning is shown as (S1). Although these drills on which thinning is performed have more effect on reduction of drilling resistance compared to drills on which no thinning is performed (see FIG. 16(c)), they do not have enough resistance reduction effect if they are used for drilling holes with man power using such as a hand drill or a drill press, and thus an operator must withstand heavy strain on his arms.
In the following patent document 2, the present applicant proposes a drill preferably used for exfoliating a spot welding part of a car's body which is made of high hardness steel sheets. This drill has two cutting blades which are symmetrical about the axis of rotation, has a tip part on which thinning is performed, and has 0.05-0.3 mm chisel width, and its thinning is performed at an inclined angle of 1-4° toward the straight line joining blade edges of both cutting blades when seen from the drill's tip side. According to the document 2, this drill has less thrust resistance at the time of drilling and requires less power applied by an operator compared to conventional drills, since its chisel width is narrow and thinning is performed at said angle. However, this drill has a rake angle shaped by thinning to more than 90° in order to correspond to the high hardness steel sheets. Therefore, the drilling force of the central part becomes weak, and at the time of drilling with a hand drill, significant power would be required while the drilling by the drill tip from a central part to a peripheral cutting blade has been performed, sinking into the workpiece. Moreover, a tip tends to chip when used since the chisel width is very narrow, and in particular, a tip of a drill made from powder high-speed steel becomes fragile and more easily chipped.
Furthermore, the following patent documents 3 and 4 also describe the drills on which thinning is performed, respectively. The drill described in the patent document 3 is used for deep hole machining and has longer thinning cutting blade for the purpose of providing enough space with a thinning pocket (a recess defined by thinning face) and smoothly discharging swarf from the thinning pocket. The drill described in the patent document 4 is a small-diameter drill for a wired printed plate and general thinning is performed on the drill preventing the drill made from cemented carbide from chipping in order to drill holes on a resin board including glass fiber. Additionally, the drill described in the following patent document 1 is a twist drill which has x shape thinning.
Thinning has been performed on every drills described in the patent documents 1, 3, and 4, but the thinning was not done for decreasing drilling resistance. Therefore, in some cases, drilling resistance may be increased but not decreased by its thinning and thus these drills cannot obtain acceptably high efficiency enough to drill holes taking advantage of manpower, such as a hand drill.